In many cases it is desirable to know the position of a moveable element within a downhole tool. This is particularly significant in a downhole flow control device where the position of the moveable element controls the flow into the well. The moveable element in these devices is typically moved by hydraulic or electric means. Without a positive position indication, it is difficult to ensure the moveable element has actually been moved to the desired position. The present invention provides an apparatus for positively determining the position of the moveable element.
In a typical hydraulically actuated intelligent well system, one or more downhole flow control devices are located in a well. These flow control devices are actuated by supplying hydraulic pressure from the surface to move a piston mechanism that in turn causes the moveable element or insert to translate to desired position. To precisely position the flow control device to the desired setting requires feedback as to its actual position. Without this feedback, derived feedback methods are used such as that described in U.S. Pat. No. 6,736,213 to try to determine this position, however the derived feedback methods are limited in their accuracy. What is needed is an actual position sensor installed on the downhole flow control device that transmits the position back to the surface. The present invention overcomes the disadvantages of not having a position indication, or using a derived method to determine the position, and provides positive feedback as to the actual position of the downhole flow control device. This invention has applications in numerous downhole tools that are actuated mechanically, hydraulically or electrically.
Magnetic sensors for determining position have been used as shown in U.S. Pat. No. 5,666,050. One feature of this application is that is senses a response to a single magnet using an individual sensor that is switched on and off. It doesn't take readings from multiple sensors to measure a magnetic field to more precisely determine the movable component location.
U.S. Pat. No. 5,732,776 shows in column 23 line 25 a proximity sensor external to a valve with no details as to the sensor construction or operation. U.S. Pat. No. 6,041,857 uses a resolver connected through a gearbox to compute translation of a sleeve in a tool. This application has limited value where motors are not used to move the downhole component. Details of the sensor appear in column 9 lines 23-46. U.S. Pat. No. 6,334,486 shows the use of position sensors while mentioning a few examples such as linear potentiometers, linear voltage displacement transducers (LVDT), resolvers or a synchro to determine position, as indicated at column 2 lines 43-45. The common feature in these references is the need to mount the position sensor to the moving element or to its driver and mounting the associated electronics that interface with the sensor in the surrounding tool body creates an opportunity for signal distortion.
U.S. Pat. No. 6,848,189 in general describes a caliper measurement device to measure the diameter of a borehole during logging operations. It consists of a curved flexible member with one end fixed and the other sliding in a track as the flexible member is flexed in and out. Sensors are used to detect the position of the sliding end of the member as it moves linearly in the track. From this information, the distance to the apex of the curved member can be calculated.
In column 5, lines 20-55 the sensor array is described. A magnet is attached to the sliding end of the flexible member, and an array of Hall-Effect or other magnetic sensors detects the movement of the magnet. The signals from all the sensors in the array are then used to calculate the position of the magnet by the centroid method.
The preferred embodiment of the present invention also centers on using an array of Hall-Effect sensors to sense the movement of a magnet installed in a moving element such as a choke insert and two or more of the sensor readings are used to calculate the position of the magnet. There are several differences between the described preferred embodiment and the '189 patent. The '189 patent is a caliper device for measuring the diameter of the borehole during logging operations. The linear measurement is an indirect way of measuring this diameter. The preferred embodiment of the present invention involves measuring directly the longitudinal movement of a downhole component such as a sliding sleeve in a choke or a flow tube in a downhole safety valve.
In the '189 patent the magnet is mounted on the O.D. of the tool and is moved along a track by flexure of the curved flexible member. The sensor array is also mounted in a housing on the O.D. of the tool, or alternately sealed in the I.D. of the tool and senses the magnet through the tool wall. In the preferred embodiment of the present invention the magnet is installed in a moveable element (choke insert) in the inside diameter or the side of the tool exposed to tubing pressure. The magnet is moved along with the entire insert as the choke setting is changed. There is no track. The sensor array can be sealed in a housing on the O.D. of the tool. The magnetic field is sensed through both the housing wall and the tool body. In alternate embodiments to the preferred embodiment, the sensor array is mounted in the outer tool body and the magnet is sensed through the tool body. The sensor array is separated from the magnet by the tool body such that there is no need for a physical connection between the array and the moving element.
In the '189 patent, column 5, lines 37-42, it states that as the magnet moves, it also rotates, and therefore the magnetic field also rotates. This effect has to be compensated for during calibration. In the preferred embodiment of the present invention, the magnet preferably does not rotate or change orientation as it moves. The orientation of the magnet's north and south poles are preferably held fixed relative to the axis of the tool as shown in FIG. 6. Compensation for magnet rotation is made unnecessary.
Finally the '189 patent uses the “centroid” technique to calculate the position from the sensor readings. This is described in column 5, lines 46-53. It utilizes the output from all of the sensors in the array to calculate the position. The preferred embodiment of the present invention uses 2 or more sensor readings to determine the position, focusing on just the outputs from the sensors that are actually responding the magnetic field to determine the position. The readings from the sensors that are not sensing the magnetic field are not used. In the example shown in FIG. 9, only readings from sensors 2, 3, and 4 are used to calculate the position as opposed to the technique of the '189 patent where readings from all 8 sensors would be used. Where the position is actually being calculated at the surface, only these 3 sensor readings shown in FIG. 9, for example, would have to be transmitted to the surface, not the readings from the entire array.